Re: [PATCH 1/2] memcg: move mem_cgroup_swappiness to memcontrol.h

[Ridong Chen <ridong.chen@linux.dev>]

On 7/14/2026 9:48 AM, Barry Song wrote:
> On Tue, Jul 14, 2026 at 9:43 AM Barry Song <baohua@kernel.org> wrote:
>>
>> On Tue, Jul 14, 2026 at 9:20 AM Ridong Chen <ridong.chen@linux.dev> wrote:
>>>
>>>
>>>
>>> On 7/13/2026 11:08 PM, Barry Song wrote:
>>>> On Sat, Jul 11, 2026 at 5:12 PM Ridong Chen <ridong.chen@linux.dev> wrote:
>>>>>
>>>>> From: Ridong Chen <chenridong@xiaomi.com>
>>>>>
>>>>> The per-memcg swappiness knob is v1-only; v2 always uses global
>>>>> vm_swappiness and ignores the per-cgroup field.
>>>>>
>>>>> Guard memcg->swappiness with CONFIG_MEMCG_V1, and move the helper
>>>>> to memcontrol.h where it belongs.
>>>>>
>>>>> No functional change for v1; v2-only kernels drop the unused field.
>>>>>
>>>>> Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
>>>>> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>>>>
>>>> Reviewed-by: Barry Song <baohua@kernel.org>
>>>>
>>>> With some nits.
>>>>
>>>>> ---
>>>> [...]
>>>>>           struct mem_cgroup_per_node *nodeinfo[];
>>>>> @@ -365,6 +366,9 @@ enum objext_flags {
>>>>>
>>>>>    #define OBJEXTS_FLAGS_MASK (__NR_OBJEXTS_FLAGS - 1)
>>>>>
>>>>> +/* Defined in mm/vmscan.c; used by mem_cgroup_swappiness(). */
>>>>> +extern int vm_swappiness;
>>>>
>>>> This is a bit unusual. I'm not sure whether mm/swap.h would be
>>>> a more appropriate place for this.
>>>>
>>> Thank you for your reply.
>>>
>>> The vm_swappiness variable is not utilized within mm/swap.c.
>>> Furthermore, since memcontrol.h does not include swap.h, retaining the
>>> extern int vm_swappiness declaration in mm/swap.h will result in a
>>> compilation failure.
>>
>> If this is the case, it still seems better to keep
>> extern int vm_swappiness in include/linux/swap.h.
>>
>> Then we don't need the comment:
>> /* Defined in mm/vmscan.c; used by mem_cgroup_swappiness(). */
>>
>> It also makes it clearer that vm_swappiness is an extern variable
>> belonging to the swap module, rather than the memcontrol module.
> 
> BTW, if mem_c_group_swappiness() and vm_swappiness are only used
> within mm/, could all of these be moved to mm/swap.h and
> mm/internal.h instead?
> 
> We are making a big effort to move many unrelated things out of
> include/linux/swap.h recently. Could you check?
> 
> https://lore.kernel.org/linux-mm/20260708-ch-swap-series-plus-folio-lru-cleanup-v9-0-2bc72b4f8730@gmail.com/

Good suggestion. Moving them to mm/internal.h makes sense. Will update 
in the next version.

-- 
Best regards
Ridong